Rats come in pinkies, fuzzies, pups/crawlers, weaned, small, medium, large, XL or Jumbo...

How many you should buy at a time depends on how many snakes you're feeding. Properly packaged, rats should hold well in a freezer for 4-6 months. Again, how many snakes you are feeding, among other things, would determine whether breeding your own is for you. Breeding rodents for feeders is a lot of work, and can get out of hand very quickly if you don't manage them correctly. It also can be much cheaper than buying in rodents, depending on the bedding and food quantities you go through. You would need a well ventilated area that doesn't absorb odor terribly, and it is a lot of cleaning. I probably spend twice the amount of time on my rodents as I do my snakes.